Nearly half a million extra voters were added to the electoral register in 2010, but as many as 6 million may still be excluded, according to figures compiled by Labour.

The extra 423,800 voters registered in the 12 months to December 2010 do not appear to be a reflection of a rising population, but a consequence of belated measures taken by the outgoing Labour government to drive up registration.

The official Boundary Commission figures were extracted by Chris Ruane, the Labour MP who has campaigned on the issue of under-registration for years. Ruane said that on the basis of conversations he had held with Experian, the marketing company, he believed the true level of non-registration was closer to 6 million than the 3.5 million normally cited.

Ruane said the surprisingly large increase in registered voters in 2010 was evidence of what can be achieved with effort, some of it driven by the desire to vote in the general election.

Among the 100 constituencies with the largest increase, two-thirds were held by Labour. Of the constituencies with the smallest rises or actual falls, only 33% were Labour.

A few constituencies saw huge increases, notably Glasgow Central, which registered an extra 9,478 voters, or 16.3%. Other increases came in Folkestone with an extra 6,583 (8.5%), Glasgow North with 4,032 (8.0 %), and City of Durham with 5,017 (7.4%). Out of 651 constituencies, increases were registered in 513.

Ruane called for a thorough review of the role of Electoral Commission (EC) in overseeing registration.

Labour gave the commission responsibility for overseeing an increase in registration in 2006, but since then the performance of electoral registration officers has been uneven. A £2.5m registration pot was ringfenced by Labour, but was under-publicised and under-subscribed. The fund was terminated in the emergency budget last June.

Ruane complains registration officers' effectiveness is measured by their ability to follow procedures, rather than on outcomes. He said: "The EC does not use any sanctions or weapons against electoral registration officers who fail to undertake real registration drives. Five years after the introduction of the new laws there are still EROs who do not conduct a door knock of non-responding households."

Overall the largest increases are in inner city areas. The groups least likely to be registered to vote according to Electoral Commission research are those living in council or social housing, or private rented accommodation, especially in seaside towns. Others include the unemployed, and those on the minimum wage.